It'd be nice to have a way to make my players use some kind of fuel. Also add an alert when you are running low on the specific kind of fuel.

experiance is a setting but it's currently global, atm you can't script it. you have to use config/SurvivalFlight/general.conf

experiance is a setting but it's currently global, atm you can't script it. you have to use config/SurvivalFlight/general.conf

It would be nice to add a way to consume the item's durability as a fuel. Is it possible?

adding any of these to general.conf, yes that is very easy.

adding a fuel requiremnt inside the actuall script is also feasable
but it would result in logical issues that in order to fix,
i'd either need to figure out how to trace the bool logic of a java Predicate
or would require fscript to be able to track the reason of a result.

i don't know if the first one is even possible.
the second is a pain to code and i'm not doing it anytime soon

You can just do whatever is easier for you. You can also not do it at all and I would totally respect that!